In recent years, quality and quantity necessary for the water for daily use and for the water for industrial use have been enhanced based on the background of the worldwide population growth, industrialization, urbanization and improvement in living standard.
For maintaining the water resources in general, there is a method wherein freshwater is prepared from sea water utilizing an evaporation method or a reverse osmosis (RO) method, a method wherein fresh water is prepared from salt-containing brine utilizing a reverse osmotic phenomenon, etc. in addition to the conventionally conducted utilization of natural water obtained from the nature. However, it has been said that resources for fresh water existing in nature are limited and further that possibility of its utilization tends to become narrower due to the affection by abnormal weather in recent years. In addition, energy for heating or pressurization is necessary for the preparation of fresh water by means of an evaporation method or reverse osmosis whereby only limited region can utilize such a means.
As another method therefor, there is a course of action wherein sewage is recycled. In the conventional sewage treatment, organic components in sewage are subjected to a decomposing treatment using activated sludge followed by subjecting to sedimentation, filtration, etc. and the treated water is discharged although it has been difficult to completely remove the bacterial group such as Escherichia coli. In MBR, the water treated with activated sludge is filtered using a separating membrane. Accordingly, in MBR, the above noxious bacterial group can be completely removed and there are also many advantages such as compactness of the equipment and easy control of the operation whereby MBR has become the technology receiving much public attention in recent years. Water separated by means of MBR is not only utilizable as the water for maintaining the life and the landscape and also as the non-potable water but also can give tap water when combined with a reverse osmosis method. In a reverse osmosis method using sea water, high pressure coping with the salt concentration is necessary. When the treated water prepared by MBR is utilized as raw water, it is now possible to prepare the water safely and also with low energy.
As such, MBR has been receiving public attention as a method for solving the water shortage which is believed to happen in future. In order to further improve this method and to establish as a highly efficient system at low cost, it is necessary to retain the separation properties of the membrane and, at the same time, to secure the water permeation properties. Characteristics which are generally demanded for a membrane to be used in MBR will be shown below.
Firstly, since a bare membrane is used by immersing in activated sludge in the MBR, its using manner is rough as compared with separation membranes in other technical fields. Therefore, physical strength resisting to the use is demanded. To be more specific, high strength and hardly-elongating membrane properties are necessary so as not to result in breakage and deformation of the membrane and lowering of the properties even when the membrane is shocked by various contaminants in the activated sludge or the trans-membrane pressure (TMP) increases by filtration.
In addition, when the membrane is used for a long period in the state of being immersed inactivated sludge, pores of the membrane are clogged by secretions generated by the activated sludge or dead bodies thereof per se, by the contaminants contained in the sludge, etc. whereby the water permeation properties lower or the pump pressure is to be raised for coping therewith. That is the biggest problem in the use of a membrane called fouling. Against this problem, there is conducted such an operation that the membrane is washed using the chemicals such as sodium hypochlorite and hydrochloric acid to solve the problem of fouling whereby the membrane is returned to a fresh state. Accordingly, it is also important that the membrane has the resistance to a solution of chemicals so that the membrane is not deteriorated by those chemicals.
However, in a washing operation using those chemicals, there are many problems in terms of economy and environment such as a stop of filtering work during the operation, cost for chemicals, troublesomeness of the work, treatment of waste liquid of chemicals, etc. Accordingly, it is the biggest matter to solve how to prevent the fouling and how to make the usable period longer so as to make the washing operation by chemicals minimum.
With regard to a method for suppressing the fouling, investigations have been eagerly carried out already. Examples of the effective method are to control the membrane structure (particularly, to control the pore size and distribution thereof) and to make the membrane hydrophilic. When the pore size has variations, there are always some pores which are easily clogged whereby it is believed that the fouling quickly proceeds from such pores. In addition, when pore size on the membrane surface is too small or when opening degree is low, sucking pressure per pore becomes high whereby the possibility of clogging is believed to become high. Another method (i.e. to make the membrane hydrophilic) is explained below. Generally, many of the substances causing the fouling (foulants) are hydrophobic. When the separating membrane is hydrophobic, the foulants are apt to be adsorbed with the membrane surface due to a hydrophobic interaction whereby it is believed that the fouling easily happens and the progress thereof is also quick.
In addition, an important point in the practical use of the membrane is that the hydrophilicity continues as long as possible. As a result thereof, it is possible to save troublesomeness and cost for making the membrane hydrophilic once again after the used membrane is washed and dried. At the same time, the anti-fouling effect continues for long time even in the actual use whereby it is possible to contribute to conservation of energy and reduction of cost.
As to the membrane for MBR taking the problem of water permeability and hydrophilization as such into consideration, there has been proposed a membrane which uses a resin of a chlorinated polyvinyl chloride (CPVC) or polyvinylidene fluoride (PVDF) type as a membrane material (for example, see Patent Documents 1 and 2). To be more specific, in the patent document 1, CPVC is dissolved in tetrahydrofuran (THF), then isopropyl alcohol (IPA, or 2-propanol) and sucrose ester are further added thereto and a nonwoven fabric of polyester is impregnated with in the above-prepared solution and dried to conduct a phase separation whereupon a microporous product is formed. Further, in the patent document 2, a solution for preparing a membrane containing PVDF having excellent resistance to chemicals, a graft copolymer of methyl polymethacrylate with polyvinylpyrrolidone, N,N-dimethylacetamide and polyvinyl alcohol was prepared, applied onto a nonwoven fabric of polyester and immersed in an aqueous coagulating bath so as to prepare a porous substrate wherein porous resin layers are formed thereon.
However, in the above membrane using the conventional CPVC, it cannot be said that pore size and distribution thereof are optimized. (Various problems pointed out in the above paragraph are worried about.) In addition, there is a problem in the hydrophilizing degree or, particularly, in retaining the hydrophilicity when used for a long period. On the other hand, when materials being resistant to chemicals other than CPVC are used, there is a limitation in a method for the preparation of membrane and it is substantially impossible to prepare a membrane by a dry method which requires little investment in plant and equipment and which allows easy preparation of membrane. For example, although the flat membrane of a PVDF resin is excellent in terms of the resistance to chemicals and of the micropore density, it is prepared by a wet method or by a thermally induced phase separation method in view of the selection of solvents and non-solvents. In a wet method, skin layers and microvoids are apt to be formed on the surface and in the inner area, respectively, of a membrane whereby it is difficult to give a product having sufficient properties and strength. In a thermally induced phase separation method, since membrane pores are formed by the changes in temperature, a strict temperature control is necessary whereby investment in plant and equipment is big and disaster risk due to the operation under high temperature is also high. Moreover, cost for polymers is high as compared with a vinyl chloride resin whereby the production cost becomes high in an industrial scale.
As to an example using other hydrophilizing agents, there has been proposed a method using cellulose into which a hydrophobic group is introduced or using hydroxypropyl methyl cellulose (HPMC) into which a hydrophobic group is introduced (see Patent Document 3). To be more specific, PVC or CPVC is dissolved in THF, then the above-mentioned cellulose derivative and a non-solvent such as IPA are added and a nonwoven fabric is immersed in the above-prepared solution and dried to conduct a phase separation whereby a microporous product is formed. According to this method, since a hydrophilizing agent is just dispersed in the membrane components, the hydrophilizing agent is apt to be eluted during a membrane washing conducted in the actual use or, particularly, during a membrane washing using chemicals whereby the effect is little in view of retention of hydrophilicity. Moreover, in the patent document 3, it is necessary to use a hydrophilizing agent in an amount of at least 3% by weight to the resin materials whereby the cost becomes high.
As an example for fixing a hydrophilizing agent on a membrane surface, there has been proposed a method wherein hydroxyalkyl cellulose is fixed on a hydrophobic ultrafiltration membrane (for example, see Patent Document 4). To be more specific, an ultrafiltration membrane formed of a sulfonic polymer is immersed in an alcoholic solution containing hydroxyalkyl cellulose and, after that, it is subjected to a treatment with an autoclave in the presence of steam or water and to a treatment containing immersion in boiling water. In this method, it is probable that thermal deformation of hydroxyalkyl cellulose is too much promoted by high temperature to lower the hydrophilic effect as will be mentioned later and the hydrophilic characteristic inherent to hydroxyalkyl cellulose cannot be fully achieved. In addition, since provision of the steps for an autoclave treatment and a boiling water treatment results in much more energy consumption, it is not a good method in view of conservation of energy and cost and, moreover, there is also a possibility that the apparatus and the steps are compelled to become complicated.
As an example of a method for modifying surfaces of various polymeric support materials, there has been proposed a method wherein a hydrophilic polymer is irreversibly adsorbed with surfaces of hydrophobic membrane (for example, see Patent Document 5). To be more specific, a polysulfone (PSf) membrane is immersed in a deionized solution of hydroxypropyl cellulose (HPC) for 16 hours and, after that, it is washed with deionized water for 16 hours. In this method, neither insolubilizing treatment nor preferred thermal denaturation for fixing the HPC to the surface of hydrophobic membrane is conducted as will be mentioned later. Accordingly, the HPC does not remain on the membrane surface but is eluted during washing.
For a purpose of imparting resistance to high-temperature sterilization such as steam sterilization, there has been disclosed an example of porous membrane which is formed of HPC and polyether sulfone (PES) and which can be spontaneously moistened (for example, see Patent Document 6). To be more specific, HPC is applied to a porous membrane formed of PES and, after that, a steam sterilization treatment is conducted. In this example, thermal denaturation of HPC is too much promoted by high temperature and there is a possibility that the hydrophilic effect lowers whereby the hydrophilic characteristic inherent to HPC cannot be fully achieved the same as in the case of the patent document 4 as will be mentioned later.
For a purpose of preventing lowering of filtering speed and clogging of a hydrophobic membrane filter, there has been proposed an example wherein a cellulose derivative is adsorbed with the membrane surface so as to modify a hydrophobic membrane to a hydrophilic one (for example, see Patent Document 7). To be more specific, a hydrophobic membrane of an aromatic polymer type is impregnated with a solution of a hydrophilic cellulose derivative at the temperature which is lower than 10° C. or more from a gelling temperature or a clouding temperature and, after that, it is washed with water at the temperature which is not lower than 20° C. or more from a gelling temperature or a clouding temperature of the solution. The content of the Patent Document 7 merely teaches that, for preventing the clogging of the membrane, HPC is subjected to a hydrophilizing treatment at the temperature which is lower than the gelling or clouding temperature of HPC.